Many proposals have been made (JP-A-55-127328, JP-A-56-2926, U.S. Pat. No. 435,404, U.S. Pat. No. 4,446,328, JP-A-59-31727, etc.) regarding the catalysts used in the preparation of methacrolein and methacrylic acid through catalytic oxidation of isobutylene or tertiary butanol in a high-temperature gaseous phase. These proposals, however, mostly concern the catalyst components and their ratios. Few of these proposals make mention of preparation process of supported catalyst, and none of them refers to particulars of the catalytic active substance to be supported, such as particle size thereof and calcination conditions for the production of such substance.
In the production of a catalyst used for the preparation of methacrolein and methacrylic acid through gas phase catalytic oxidation of isobutylene or tertiary butanol with molecular oxygen in a fixed bed reactor, since this reaction is exothermic type, it is desirable to bulk up the catalytic active substance while controlling the thickness of the catalyst layer to avoid the undesirable rise of temperature due to build-up of heat in the catalyst layer. Control of the thickness of the catalyst layer is also desirable for inhibiting the consecutive reactions of the product. Therefore, use of a supported catalyst having a catalytic active substance supported on a carrier and the controlled thickness of the catalyst layer often prove to be favorable for the selectivity of the objective product.
JP-A-2-25443 discloses a production process of a supported catalyst used for the preparation of acrolein, according to which a catalytic active substance precursor ground to a particle size of about 400 .mu.m is supported on an inert carrier. JP-A-58-930 discloses a production process of a supported catalyst used for the preparation of acrolein or methacrolein, and in an embodiment thereof, there is shown a method for supporting a catalytic active substance precursor ground to a particle size distribution of around 2-80 .mu.m, with the mode of the particles falling in a range of about 10-30 .mu.m, on an inert carrier. According to this method, the primary calcination for obtaining a catalytic active substance precursor is carried out at around 500.degree. C., and the secondary calcination conducted after the supporting operation is carried out at a temperature which is approximately 20.degree. C. higher than the primary calcination temperature. The supported catalysts obtained from these methods are still unsatisfactory for industrial use in such respects as catalytic activity, selectivity of the objective product and mechanical strength, and further improvements have been desired from the industrial standpoint.